Axial piston type transmissions comprising an axial piston system operating as a pump and a second axial piston system acting as a motor are well known. The axial pistons of the pump move in a cylinder block that is rotated by an input shaft. The axial pistons engage the surface of a swash plate that is inclined with respect to the axis of rotation of the cylinder block, causing the pistons to reciprocate in the cylinders as the block rotates relative to the swash plate. The reciprocating pistons pump fluid through a rotary valve which divides the cylinders into two groups, the valve forming a high pressure output port for one group and a low pressure input port to the other group. The angle of the swash plate surface controls the stroke of the pistons. The motor pistons engage a second swash plate. The ratio of input and output speed is equal to the ratio of the stroke volumes of the motor and pump and is varied by varying the inclination of the swash plates. In some transmission systems the swash plates rotate with the drive shaft, and part of the power is transmitted mechanically and part of the power is transmitted through the hydraulic fluid. For example, if the swash plate associated with the motor is perpendicular to the axis of rotation of the drive shaft, no hydraulic fluid is pumped and the drive shaft and the output shaft are rigidly coupled together with all the power being transmitted by mechanical coupling with a 1:1 speed ratio, or direct drive. A transmission system of this type is described, for example, in U.S. Pat. No. 3,504,492.
One problem with such systems in which power is partially transmitted mechanically through the swash plates is that large thrust loads are encountered on the swash plates. As a result, the mounting for the swash plates requires very sturdy mechanical parts and bearings. Also a large force may be required to shift the angle of the swash plates.